Many people with visual disorders never have those disorders improved. Corrective lenses establish normal visual acuity for many people, but the underlying visual disorder persists. Lenses alone cannot compensate for some disorders.
Presently, treatment of such disorders as strabismus, amblyopia, myopia, and accommodative insufficiency is accomplished through widely varying techniques. Strabismus, known as cross-eyes, is the condition where the visual axes of the eyes cannot be directed to the same object at the same time and is due to a lack of muscular coordination. One eye turns either vertically or horizontally relative to the other eye. Strabismus is typically caused by genetics, injury or disease which results in a misalignment of one or more of the six pairs of ocular muscles controlling the eyeball. Double vision or suppression of the strabismic eye often results.
Strabismus can be treated through surgery. Surgery is expensive, invasive, and has a surprisingly low rate of effectiveness. Over 85% of the operations do not provide effective treatment for strabismus. Most surgery simply treats strabismus cosmetically by physically reorienting the eyeball, but the patient is still not using both eyes together as a unit. Any underlying amblyopia which frequently accompanies strabismus remains untreated.
Amblyopia involves reduced vision in one eye, typically because of mental suppression. The brain partially or completely ignores optic nerve signals from that eye. Amblyopia is frequently induced by strabismus or by a high refractive error in only one eye.
Amblyopia results in poor visual efficiency due to decreased stereopsis and poor distance judgment. This condition is frequently treated by patching the good eye to force the brain to utilize the amblyopic eye. Recent studies indicate, however, that such treatment can induce amblyopia in the patched eye.
Myopia, known as nearsightedness, is a refractive disorder where the eye focuses incoming light to a point in front of the retina. Blurred distance vision results because the refractive system of the myopic eye remains excessively convex, that is, it converges light too quickly, when distant objects are viewed. Some progress has been made with biofeedback techniques which attempt to train the muscles controlling the shape of the lens of the eye. A biofeedback instrument, however, typically costs upward of $15,000. Myopia is correctable with lenses but is not improved. The American Optometric Association estimates that two to four percent of children under six years of age have amblyopia and that 40% of the total population of the United States are likely to develop myopia.
Accommodation is an involuntary adjustment of the eye to focus the image of an object on the retina. In accommodation for near vision, the ciliary muscle contracts to reduce tension on the lens and allow it to become steeper, that is, to become more convex. Several factors reduce the accommodative mechanism. During the normal aging process the lens becomes stiffer. Environmental, genetic and other factors can increase lens stiffness or weaken the ciliary muscle. These result in headaches and discomfort when working on near tasks such as reading or focusing on a computer screen.
The field of vision training involves therapy to improve conditions such as strabismus and amblyopia. The optometrist prescribes visual tasks to be practiced under controlled conditions. In-office training is recommended for most patients and especially those with strabismus and amblyopia. Home training is particularly not effective for children since they lose interest in the training and have difficulty monitoring their progress.
There are several large instruments for analyzing strabismus, amblyopia, myopia and accommodation disorders. Two of such devices are the VS-II Vision Screener and the Opthalmic Telebinocular, available from Keystone View, Davenport, Iowa. The Vision Screener occupies over one-half cubic foot of space, weighs more than ten pounds, and must be plugged into a wall outlet. It has a viewing head with a forehead rest and a lens system with one viewing distance of 15 inches and another of 6 meters, equivalent to optical infinity. Static targets on test slides are illuminated by reflected light. The vision screener tests for visual acuity, phorias, fusion, depth perception and color perception. Images for the left and right eye are successfully presented on a rotating target drum.
The Opthalmic Telebinocular is much larger and again uses static targets. An occluder is provided for each eye to permit monocular testing or vision training. A patient is quickly bored when vision training is conducted on this and similar instruments. Since improvements in vision depend heavily on the motivation of the patient and the frequency of therapy, progress is typically quite slow. Further, the training sessions must be conducted in the office of the optometrist or ophthalmologist.